Super abrasive grinding wheels

ABSTRACT

The invention relates to a superabrasive grinding wheel comprising very hard abrasive particles and a binder with a metallic matrix, which also contains &#34;pore-forming&#34; adjuvants, notably hollow beads made of ceramic. 
     This grinding wheel may be used notably for the machining or grinding of glass articles, especially for the grinding of the edges of glass sheets.

The present invention relates to "superabrasive" grinding wheels. Thisterm is used to designate grinding wheels of very high abrasivity, basedon very hard abrasive particles, especially of diamond or of cubic boronnitride, and on a binder which enables these particles to be retainedand maintained in place.

This binder may be of three types: it may be a resin, especially apolyimide or phenolic resin. It may also be a vitrified binder in theform of a ceramic matrix of the alumina, alumina-silica orcarbide-silica type. The binder may also be based on a metallic matrix,and it is with this third type of binder that the invention is moreparticularly concerned, because it exhibits a particularly advantageousmechanical strength.

A constant difficulty in the grinding process is proper removal of thedust or other waste products to which the operation gives rise. To dothis, a medium, generally water, is conveyed in the direction of thegrinding wheel in order to entrain this dust and also, of course, tocool the grinding wheel. However, since the grinding wheelsimultaneously attacks a whole portion of the surface of the articlebeing treated, the coolant liquid has difficulty in moving forwardtowards the whole part being treated. Some accumulation of the wastethen takes place. In the case of the treatment of articles made ofglass, for example, the grinding is thus systematically accompanied bythe formation of a layer of glass paste which tends to oppose the actionof the grinding wheel and slows down the grinding operation, making itnecessary to perform a number of runs.

The objective of the present invention is an improved type ofsuperabrasive grinding wheels with binder with metal matrix permitting abetter management of the problem of the dust and other waste.

The subject of the invention is a superabrasive grinding wheel based onvery hard abrasive particles of the diamond or cubic boron nitride typeand on a binder with a metallic matrix, and which additionally containspore-forming elements. Included under this heading are elements thefunction of which is to create some porosity within the binder of thegrinding wheel.

Until now, on the contrary, there has been a tendency rather to employmetallic binders which are as dense as possible, this being in anattempt to slow down the erosion of the grinding wheel as much aspossible. However, it has been found, surprisingly, that it is in factquite advantageous to have a certain porosity "content" in the metallicmatrix of the binder, because this porosity very significantly improvesthe problems of accumulation of waste during the abovementionedgrinding, and even result sin lengthening the lifetime of the grindingwheel.

These pore-forming elements have to be chosen as a function of theprocess of manufacture of the grinding wheels. In particular, they mustbe capable of withstanding the pressure and the appropriate temperature.This is the reason why use is preferably made of elements in the form ofhollow ceramic beads, especially based on silicon and/or aluminium oxidesuch as alumina or mullite. Mullite is an aluminium silicate of the2SiO₂ --3Al₂ O₃ type. These beads are advantageously chosen with anouter diameter of between 1 micron and 3 mm, especially between 100microns and 1 mm. Their walls preferably have a thickness of between 2and 8, especially between 4 and 6 micrometers.

These pore-forming elements are preferably added to the grinding wheelin a proportion of 1 to 80% of the total volume of the grinding wheel,especially between 5 and 50%, or approximately 30% of the said volume.

The way in which these pore-forming elements function is as follows: asthe grinding wheel becomes worn, the hollow beads situated at thesurface progressively break and the surface of the grinding wheel thenbecomes pockmarked; the glass paste can then accumulate in these hollowswithout interfering with the progress of the grinding. In addition, thecoolant liquid can move forward continuously at the interface betweenthe grinding wheel and the article being treated and can thus penetrateright through the bottom of these hollows, expel the glass paste--or anyother type of dust--which is thus removed in order to be finallyreturned via the bottom of each hollow formed by a bead.

Furthermore, the coolant liquid thus acts on a much larger area thanmerely the surface of the rim of the abrasive grinding wheel, permittinga direct cooling to a depth of the order of the diameter of the beads,which correspondingly increases the efficiency of the cooling and, as aresult, slows down the wear of the grinding wheel.

It is therefore understandable that it is advantageous for the wall ofthe hollow beads to be very thin, insofar as this facilitates theirbreaking, which is what is being primarily sought after in theinvention. The bead content of the grinding wheel has to be modified asa function of the type of articles which it will be necessary to grind.

With regard to the metallic matrix of the binder, this can be chosen asa function of the applications for which the grinding wheel is intended.Cobalt is widely employed, and so is bronze, silver--which exhibits thespecial feature of being relatively ductile, iron or copper.

Different additives, especially such as tungsten carbide, may be addedto this matrix in order to increase the erosion resistance of thegrinding wheel.

With regard to the abrasive particles of the grinding wheel, thesepreferably correspond to 5 to 60% of the total volume of the grindingwheel, especially from 10 to 30% of the said volume. They may have arounded shape or be needle-shaped. Their size is evaluated with the aidof a standardized coding of the European manufacturers of abrasivescalled the FEPA Code; in this case, a grain size according to this codeof between 4 and 1182 is chosen, which corresponds to a particle "meandiameter" of 4 microns to 1.100 mm. A grain size between 40 and 90 ispreferably chosen. There again, everything depends on the future use ofthe grinding wheel, the finest abrasive particles making it possible toobtain the most highly polished surface quality of the workpiece to beground.

The grinding wheel according to the invention is advantageously employedfor machining or grinding glass articles, especially for grinding theedges of glass sheets.

An example of a grinding wheel according to the invention is produced asfollows: a grinding wheel is manufactured according to the known methodsof manufacture with 15% by volume of diamond particles of 91 grain sizeand 30% by volume of hollow mullite beads with an outer diameter ofapproximately 0.5 mm and wall thickness of approximately 5 micrometers,with a cobalt binder. The results are conclusive: grinding of the edgesof glass sheets is facilitated by better removal of the waste and bettercooling of the grinding wheel, additionally entailing an increase in thelifetime of the grinding wheel of nearly 30% compared with a similargrinding wheel without alumina beads.

I claim:
 1. Superabrasive grinding wheel comprising very hard abrasiveparticles of diamond or cubic boron nitride and a binder with a metallicmatrix and pore-forming elements, wherein the pore-forming elements arehollow beads of ceramic, the walls of which have a thickness of between2 and 8 micrometers, whereby breakage of the walls of the hollow beadsand waste removal is facilitated.
 2. Grinding wheel according to claim1, wherein the hollow beads have an outer diameter of between 1 micronand 3 mm.
 3. Grinding wheel according to claim 2, wherein the hollowbeads have an outer diameter between 100 microns and 1 mm.
 4. Grindingwheel according to claim 1, wherein the pore-forming elements are addedin a proportion of a volume of 1 to 80% of the total volume of thegrinding wheel.
 5. Grinding wheel according to claim 4, wherein thepore-forming elements are added in a proportion of a volume of between 5and 50% of the total volume of the grinding wheel.
 6. Grinding wheelaccording to claim 5, wherein the pore-forming elements are added in aproportion of a volume of approximately 30% of the total volume of thegrinding wheel.
 7. Grinding wheel according to claim 1, wherein themetallic matrix of the binder is selected from the group consisting ofbronze, silver, cobalt, iron, copper and mixtures thereof.
 8. Grindingwheel according to claim 1, wherein the binder with metal matrixcontains a tungsten carbide additive.
 9. Grinding wheel according toclaim 1, wherein the abrasive particles correspond to 5 to 60% of thetotal volume of the grinding wheel.
 10. Grinding wheel according toclaim 9, wherein the abrasive particles correspond to 10 to 30% of thetotal volume of the grinding wheel.
 11. Grinding wheel according toclaim 1, wherein the abrasive particles are rounded in shape orneedle-shaped, with a grain size of between 4 and 1182 in FEPAstandardized coding.
 12. Grinding wheel according to claim 11, whereinthe abrasive particles are rounded in shape or needle-shaped, with agrain size of approximately 40 to 90 in FEPA standardized coding. 13.Grinding wheel according to claim 1, wherein said hollow ceramic beadsare made of alumina or mullite.
 14. Grinding wheel according to claim 1,wherein the hollow beads have a wall thickness between 4 and 6micrometers.
 15. A method of grinding glass articles, comprisinggrinding a glass article with the grinding wheel of claim
 1. 16. Themethod of claim 15, comprising grinding an edge of a glass sheet.